Seedlot vigor is the percentage of viable seeds within a given seedlot that possess sufficient growth potential to successfully progress through plant emergence under normal germination environmental conditions. Seedlot vigor is thus the percentage of seeds within a seedlot that possess the potential to become marketable plants. All other factors considered, this represents the maximum stand a seedlot can be expected to produce.
Seedlot vigor is often tested using plug trays. A plug tray consists of a series of small cells for sowing seeds typically for fruits, vegetables, and ornamental plants. An accurately sown crop in a plug tray consists of having exactly one seed per cell with good seed placement in each cell which contains adequate quality plug media. There are four stages through which a seed can progress in a plug tray. Stage 1 starts at the point when a seed has been sown, and ends when a radicle, i.e. embryonic root of a seedling, emerges from the seed coat. Stage 2 continues until emergence of the actual plant from the media. Stage 3 covers the growing of the plant until the plant is ready for sale. Stage 4 occurs during the time when a plant is being "finished" or hardened. Assuming a crop has been accurately sown, the final stand of marketable plants expected in a plug tray is dependent in large part on two factors: Seedlot vigor and environment.
An adequate environment must be provided to realize the maximum stand potential of a given seedlot. This includes proper levels of light, temperature, nutrition, water and gases. Additional cultural practices including chemical selection, application, and so forth, must also be managed in a logical and efficient manner.
In the past, the determination of seedlot vigor has been considered inconsistent and unreliable. The accepted seed testing standard set by the American Organization of Seed Analysts (AOSA) involves placing a prescribed number of seeds on blotter paper in a controlled environment for a certain length of time. By providing a controlled, standardized environment during the germination process, concerns with environment are reduced to a minimum. The final data obtained from this system consists of subjective counts of good seedlings collected at specified times.
As subjective counts are often found to be inaccurate, plug producers have turned to plug tray tests to measure the percentage of seeds within a seedlot capable of becoming marketable plugs, i.e. the final stand potential of a seedlot. One common goal of both the plug tray and blotter paper tests is to avoid having to identify the actual cause of poor vigor and just determine the percentage of seeds within a given seedlot which are satisfactory for use in commercial production. An in-house plug tray is generally able to distinguish between seedlots in a specific location which are capable of producing a final stand greater than 90%, for example, from those not likely to surpass 80%. However, even a well-replicated plug test cannot be expected to reliably predict final plug stands due to normal variation in cultural practices and germination environment. The plug tray test, however, has gained much acceptance because it automatically screens out the lowest vigor seeds because they simply do not emerge.
One problem with the plug tray test is that it cannot distinguish between seedlot vigor and germination environment. For example, if a given seedlot produces an 80% final stand, it does not necessarily follow that this seedlot possesses a total stand potential of 80%. It is possible for more than 80% of the seeds within this seedlot to have adequate vigor, but due to environmental problems, the final stand is only 80%. In addition, plug tray tests require expensive facilities and are labor and time consuming, often requiring 21 to 28 days for completion.
Another commonly used method involves the use of a stress test, in which it is presumed that if a seed can germinate satisfactorily under less than ideal conditions, it should be considered vigorous. One shortcoming with this approach, whether conducted on blotter paper or in a plug tray, is that stress tests have been found to be unreliable. One repercussion of testing seeds in conditions significantly different from normal production is often very inconsistent results.
Systems trying to exploit factors related to seed composition, such as levels of protein, ATP or RNA synthesis and chromosome or membrane degradation have encountered similar inconsistencies. A common characteristic of these tests is an attempt to isolate one component from a very complicated biochemical process. The hypothesis is that this one narrow factor changes in direct proportion with the ability of a seed to produce a marketable plug. Such biological systems, however, are generally too complex to allow such narrow tests to consistently and reliably predict such a general outcome.
Commercial plug production has essentially become a manufacturing process with growers facing ever-increasing demands to maximize the final stand of marketable plugs in each plug tray as efficiently as possible. Financially, seed is a major production input and seed vigor directly affects final stand.
Thus, what is needed is an efficient and economical method and apparatus which utilizes appropriate environmental conditions, measures a parameter which correlates highly with the characteristic of interest, and employs an appropriate experimental design, including sample size and test replication in order to insure that test results match actual final stands obtained from normal commercial production.